A considerable amount of interest has recently been generated in permanent magnets made from alloys containing iron, boron along with neodymium and/or praseodymium. Such alloys typically contain about 6 to about 50 atomic percent neodymium and/or praseodymium, about 1 to about 10 atomic percent boron and about 50 to 90 atomic percent iron. Nickel or cobalt may be used to replace some of the iron. The alloys may also contain small amounts of such other elements as gallium, aluminum, silicon and manganese. In one particular species of such magnets, for example, ribbons of the alloys are made by a rapid solidification process (i.e., melt-spinning) in which the molten metal is ejected through an orifice onto the surface of a rapidly rotating disc (i.e., about 19 meters/sec) where the cooling rate can be as high as 10.sup.6 .degree. K./sec. The precise microstructure of the alloy depends on the cooling rate which is readily changeable by varying the surface velocity of the rotating disc. At high disc speeds (i.e., high quench rates), the alloys are amorphous with negligible intrinsic coercivity. At low speeds (i.e., low quench rates), the alloys contain large grains with low coercivity. Alloys having high coercivity comprise substantially spherical grains of Nd.sub.2 Fe.sub.14 B (i.e., between 20 and 200 nm in diameter). The ribbons are coarsely ground and the particles hot-pressed at about 970.degree. K. and about 100 MPa to form dense ingots wherein the grains of Nd.sub.2 Fe.sub.14 B are surrounded by a thin layer of a Nd-rich, B-lean amorphous phase at the grain boundaries. Another technique for making magnets from such alloys involves sintering alloy powders which have been oriented and pressed in a magnetic field. Magnets made from such alloys are hard and brittle and require expensive diamond or cubic boron nitride tools for conventional machining. Even then, they are easily chipped and fractured.
Electrolytic machining is a well known process for shaping hard or brittle alloys which are otherwise difficult to machine. Electrolytic machining techniques are generally classified into one of two basic categories, namely electrolytic grinding and electrochemical machining and are essentially selective electrochemical corrosion processes. Electrolytic grinding is generally suited to metal removal operations ordinarily performed by cut-off wheels, saws, and grinding or milling machines and uses equipment similar in appearance to conventional cutting apparatus except for the electrical accessories. About 95 percent of the metal removal results from electrolytic rather than mechanical action. In its simplest form, electrolytic grinding is a process wherein an anodic workpiece is bathed in an electrolyte and an electric current passed therethrough to dissolve the surface of the workpiece. The resultant film of insoluble salts or oxides formed on the surface of the workpiece is then scraped away by a rotating cathodic grinding wheel. Electrochemical machining, on the other hand, relies solely on reaction product removal by means of electrochemical action and a rapid circulation of electrolyte in the region being cut. These processes are well known and are described in more detail in Kirk & Othmer, Encyclopedia of Chemical Technology, 3d Ed., Vol. 8, 751-762, John Wiley & Sons, 1979.
One of the problems in electrolytic machining processes is the uncontrolled anodic dissolution of the workpiece in unwanted areas resulting in undesirable tapering of holes, rounding of edges, and the like. This undesirable condition is often called "overcut". Overcut can occur even in low current density areas of the workpiece which are fairly well removed from the cathode. Overcut in low current density areas of the workpiece remote from the cathode can be reduced by the use of passivating electrolytes, such as sodium chlorate or perchlorate, which form passive films in the low current density regions of the workpiece such as described in LaBoda U.S. Pat. No. 3,669,858. The films are destroyed in the high current density regions where rapid metal removal rates are achieved along with very smooth machined surfaces. Electrolytic machining of neodymium/praseodymium-iron-boron alloys using sodium chlorate electrolytes, however, results in overcutting apparently due to the presence of the highly active neodymium and/or praseodymium in the alloys.
Accordingly, it is an object of the present invention to provide a unique passivating electrolyte tailored specifically for the electrolytic machining of neodymium/praseodynium-iron-boron alloys which electrolyte results in machined parts having minimal overcutting and smooth surface finishes. This and other objects and advantages of the invention will become apparent from the following description thereof.